1. Field of the Invention
The present invention relates generally to fabricating flash memory and in particular the present invention relates to using oxygen free radical process cell oxidation to improving the performance of flash memory.
2. Description of the Prior Art
Nonvolatile memory devices include flash EEPROMs (electrical erasable programmable read only memory devices). FIG. 1 represents the relevant portion of a typical flash memory cell. The memory cell typically includes a substrate 10 and a thin gate dielectric layer 20 (commonly referred to as the tunnel oxide) formed on the surface of the substrate 10, two stacked gate structure 70 overlying the tunnel oxide 20. The stacked gate 70 also includes a first polysilicon layer as a floating gate 30 which overlies the tunnel oxide 20 and an interpoly dielectric layer 40 which overlies the floating gate 30. Lastly, a second polysilicon layer as a control gate 50 overlies the interpoly dielectric layer 40.
As shown in FIG. 1, the stacked gate structure 70 is substantially etched away using conventional etching techniques. However, a problem often occurs at this step involving formation of polystrings 60. Poly stringers 60 result from incomplete removal of polysilicon layer 30 from the unmasked portions of the wafer during etch. These remaining portions of polysilicon layer 30 material are known as polysilicon layer stringers 60 as shown in FIG. 1, which may result in electrically shorting adjacent memory cells. In the other words, the polysilicon layer 30 etching step serves in part to isolate one memory cell from another. However, if a portion of the polysilicon layer 30 is not etched away forms a conductive path (e.g., poly stringer 60) from one memory cell to another, the memory cells will become electrically shorted.
As shown in FIG. 2, the interpoly dielectric layer 40 has a number of important functions including insulating the control gate 50 from the floating gate 30. Accordingly, it is desirable to form a high quality interpoly dielectric layer 40. The interpoly dielectric layer 40 is often a multilayer insulator such as an oxide-nitride-oxide (ONO) layer 40 having two oxide layers 42 and 46 sandwiching a nitride layer 44. The thick of bottom oxide layer 42 is about 43 angstrom and top oxide layer 46 is about 62 angstrom. The thick of nitride layer 44 is about 59 angstrom. If the oxide layer 42, 46 is too thick, the required programming voltage increases undesirably.
As shown in FIG. 3, in conventional cell oxidation method, there are dry oxidation, wet oxidation and dry RTO method. They surrounded the surface of the cell to formed thin oxide layer 80 and thick about 100 angstrom. The object of the thin oxide layer 80 provided an insulation protection of the cell. Moreover, the thick of oxide layer is slight, the poly stringer 65 is still exist. The poly stringer 65 which may result in electrically shorting adjacent memory cells. However, if a portion of the poly stringer 65 is not oxidation away formed a conductive path from one memory cell, the memory cells will become electrically shorted. The result is reduced performance of flash memory cell.
As shown in FIG. 4, but in those oxidation methods, no matter using dry oxidation, wet oxidation or dry RTO method the problem of serious interpoly dielectric layer 40 encroachment (to formed oxide layer) can""t on a valid time programming erased of flash memory. In flash memory, the gate coupling ratio (GCR) value will decrease apparently when the thickness of cell oxidation is increased. This is because of the oxidation encroachment issue between inter poly dielectric layer 40 and polysilicon layer 30, 50 interface. As shown in FIG. 4, the interpoly dielectric layer 40 comprised bottom oxide layer 42, nitride layer 44 and top oxide layer 46. When running oxidation process of the cell, serious encroachment issue 42-1,46-1 between bottom oxide layer 42 and first polysilicon layer 30, top oxide layer 46 and second polysilicon layers 50. The result is thick of ONO layer (interpoly dielectric layer) 40 increased, the required programming voltage increases undesirably or reduced operating speed.
The method of cell oxidation by dry oxidation process is grown at 875xc2x0 C. on furnace. The process condition are temperature about 875xc2x0 C., oxygen gas flow about 10000 sccm and time about 42 minutes 30 seconds. The result diagram as shown FIG. 4, the thick of cell oxide layer 80 about 100 angstrom, it is found that a serious encroachment after cell oxidation process is observed. The encroachment oxide is grown between ONO layer 40 and first, second polysilicon layers 30, 50 interface. This encroachment issue 42-1,46-1 will increase the thickness of ONO layer 40 and decrease gate coupling ratio. Lower gate coupling ratio will degrade channel erase speed.
The method of cell oxidation by wet oxidation process is grown at 820xc2x0 C. The process condition are the temperature about 820xc2x0 C., the oxygen gas flow about 4000 sccm, the hydrogen gas flow about 7200 sccm, time about 7 minutes and 10 minutes by annealing with nitrogen gas 15000 sccm. From this method result, diagram same as FIG. 4, the thick of cell oxide layer 80 is about 70 angstrom. It is found that cell oxidation by wet oxidation is much more serious ONO encroachment issue 42-1, 46-1 than that by dry oxidation. Even if the thickness of wet oxidation only 70 angstrom, the encroachment oxide grown between ONO layer 40 and polysilicon layer 30, 50 interface is very thick.
The method of cell oxidation by dry RTO process is grown at 1100xc2x0 C. The process condition are temperature about 1100xc2x0 C., time about 140 seconds. From this method result, the thick of cell oxide layer 80 is about 120 angstrom. It is found that cell oxidation by RTO also has encroachment issue 42-1, 46-1 like dry oxidation, the diagram same as FIG. 4.
In the prior art of cell oxidation method, no matter using dry oxidation, wet oxidation or dry rapid thermal oxidation (RTO) method the serious ONO encroachment to formed. Then the two oxide layer of ONO layer are thicker, the required programming voltage increases undesirably. The result is reduced performance of flash memory.
In this invention, they will demonstrate a powerful oxidation method to solving ONO encroachment issue by oxygen free radical process. Scaling down the thickness of encroachment oxide of cell oxidation will improve the gate coupling ratio, however, poly stringer and repaired capability of etching damage are other serious after etched. In order to oxidation poly stringer fully, it is necessary to use thicker cell oxidation process.
In accordance with the present invention, it is a main object of this invention to form cell re-oxidation is described which uses an oxygen free radical process.
It is another object of this invention by using oxygen free radical process cell oxidation improved ONO encroachment between oxide layer and polysilicon layer interface.
It is another object of this invention by using oxygen free radical process cell re-oxidation to eliminate poly stringer.
It is another object of this invention by using oxygen free radical process cell oxidation increased operation efficiency of flash memory.
It is another object of this invention by using oxygen free radical process cell oxidation modified defects after etching process.
It is another object of this invention by utilizing a low thermal budget process for performing a cell re-oxidation, and thus, the short channel effect in a semiconductor structure can be reduced.
In this invention, the method for improving the performance of flash memory. First, a substrate is proved. A tunnel oxide layer is formed on the substrate. There two gate structure are formed on the tunnel oxide layer. The gate structure comprising a first polysilicon layer as a floating gate, an interpoly dielectric layer such as ONO layer on the floating gate, a second polysilicon layer as a control gate on the interpoly dielectric layer. Moreover, the poly stringer is exit between the gates, wherein the poly stringer is unmovied after etched. Next, the oxygen free radical process cell oxidation is processed. In this invention by using activity of oxygen free radical interaction with cell surface quickly in the low pressure. The results ONO encroachment is very slightly then improvement of 6% GCR with oxygen free radical process cell oxidation can increase operation speed by more than 5 times and eliminated poly stringer.